In current reconfigurable optical add/drop multiplexers, ROADM, flexibility is only provided for handling wavelength channels coming from the network (by-pass wavelengths) that can be remotely configured at each node to be routed from any direction to any direction. At the end points the add and drop wavelengths are rigidly assigned to a fixed direction and a fixed colour/wavelength port can only be reconfigured manually.
Next generation ROADM will need to be more flexible, with colorless, directionless and contentionless operation so as to extend the flexibility and automation to the end points where transponders are connected a communications network node. Colorless, directionless and contentionless operation will enable the configuration, without any manual intervention, of add/drop wavelength channels to/from any direction (directionless operation), independent of the transponder wavelength (colorless operation) and enabling multiple signals with the same wavelength to be handled by the same add and drop structure (contentionless operation).
Dynamic colorless, directionless and contentionless add/drop access will give network operators the ability to optimize network resource utilization, eliminate manual intervention, and support re-routing functions in case of faults in a cost effective way. To add this flexibility to existing ROADMs that use free space optics based 1×N wavelength selective switches, WSS, for optical line switching, new optical switching node architectures have been proposed based on the use of an additional block referred to as an Add and Drop Switch and Aggregator block (ADSA).
Three main potential implementations of an ADSA have been proposed:
1) An ADSA based on arrays of 1×N WSS in conjunction with arrays of 8×1 space switches, as disclosed in P. Colbourne et al, ‘ROADM Switching Technologies’ Optical Fiber Communications, OFC, 2011, paper OTuD1.
2) An ADSA based on a distribute-switch-select architecture in which splitters/combiners are used in conjunction with 1×N optical switches and tunable filters, as disclosed in FIG. 5 of S. Gringeri et al: ‘Flexible Architectures for Optical Transport Nodes and Networks’ IEEE Communication Magazines, July 2010.
3) An ADSA including a high port count optical cross connect, OXC, based on free space optics and micro-electro-mechanical system, MEMS, technology, as disclosed in R. Jensen at al, ‘Colourless, Directionless, Contentionless ROADM Architecture Using Low-Loss Optical Matrix Switches’, European Conference on Optical Communications, ECOC, 2010 paper Mo.2.D.2.
Each of these proposed ADSAs have associated problems. ADSA 1) has low optical loss but has the disadvantage of high cost, room occupancy and complexity due to the high number of WSS in the array that increases with the transponder count. ADSA 2) has the disadvantage of high optical loss that increases with the transponder count due to signal distribution and switching. This leads to an increase in ROADM cost, footprint and power consumption, due to the required levels of optical amplification. ADSA 3) has the disadvantage of high cost and large footprint due to the use of 3D MEMS technologies, plus the complex control circuitry required by that technology and the need to install the ADSA with the maximum possible sized switch matrix even if a lower number of transponders are initially being used.
Further implementations of an ADSA for optical communications networks are disclosed in PCT application no. PCT/EP2011/070543. A first ADSA architecture disclosed in PCT/EP2011/070543 comprises a first plurality of drop optical switches for dropping wavelength channels arranged in a first switching matrix and a second plurality of add optical switches arranged in a second switching matrix for adding wavelength channels. A second ADSA architecture disclosed in PCT/EP2011/070543 comprises a plurality of add/drop optical switches arranged in a switching matrix. The drop switches and the add/drop switches may comprise microring resonators or Mach-Zehnder interferometer based broadband photonic switches. The add switches may comprise wavelength tuneable photonic switches.